Ferrite memory cores



United States Patent 3,235,504 FERRITE MEMORY CORES Robert S. Weisz,Pacific Palisades, Calif, assignor to Electronic Memories, Incorporated,Los Angeles, Calif., a corporation of California No Drawing. Filed Jan.16, 1963, Ser. No. 251,715 7 Claims. (Cl. 252-625) This inventionrelates to ferrite magnetic core material used in the fabrication ofmagnetic core storage systems, and more particularly to an improvedferrite core material composition.

Attempts to switch presently commercially available ferrite cores atrates such that a full duty cycle occurs within 0.5 microsecond or less,are not quite successful. Cores which are considered low-zinc(containing less than 8 percent zinc oxide) in the 30 mil size requirein the neighborhood of 500 milliamperes to switch at 0.45 microsecond.Driving these cores at repetition rates on the order of 0.5 megacyclecauses the cores to heat up and become unstable, and therefore unfit foruse as a storage device. The commercially available cores which arehigh-zinc (containing in excess of 8 percent zinc oxide in thecomposition), although less prone to overheat due to a lower coerciveforce, are undesirable for other reasons. These high zinc coresinvariably have a rounded hysteresis loop, and associated with this theyhave a high disturb zero response voltage and a high delta noise.

Because of the foregoing drawbacks of presently available cores, fastswitching ferrite memory systems (those having one microsecond or lessread/write cycle), are generally built using a word select mode ofoperation instead of a coincident current mode of operation, because inthe word select mode of operation there is less disturb current flowingduring the operation thereof. However, a word select memory is not themost desirable type since in its construction it uses a considerablylarger number of semi-conductor devices than are used in a coincidentcurrent mode type of memory.

An object of this invention is the provision of a novel formulation forferrite memory cores which can be operated at full duty cycles at 0.5microsecond or less without heating up and becoming unstable.

Another object of the present invention is the provision of a new andimproved magnetic ferrite core formulation.

Yet another object of this invention is the provision of a uniqueferrite memory core formulation which results in a core having a lowernoise and lower heating when switched at a higher repetition rate thanpresently available cores.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe following description.

It has been found that satisfactory performance at full duty cycles of0.5 microsecond or less is achieved by using compositions for theferrite cores which may be expressed where x varies from .05 to .15 mol.In addition up to .06 or .07 mol of ZnFe O can replace an additionalportion of MgFe O Ferrite cores having a formulation falling within theranges indicated had stability even though switched at repetition ratesexcess of 0.5 megacycle and also these cores had a much lowerundesirable noise characteristic.

In addition to being in the above-compositional range,

3,235,504 Patented Feb. 15, 1966 a further necessary condition forproducing fast switching ferrites in control of the crystal size. Thesmaller the crystal size the higher the coercive force, but the fasterthe switching time. Crystal size control may be accomplished by usingiron oxide and other constituents of low reactivity or by calcining at ahigher temperature than customary and by grinding for a shorter thanusual time after calcining. The control of crystal size using thesetechniques is generally known.

As an illustration of a formulation used for the cores, a typical methodfor processing the formulation to achieve cores having the desiredproperties, a formulation consisting of 96 grams of MgCO 55.3 grams MnCO112 grams of Fe O and 22.6 grams of SnO was mixed with distilled waterand then placed in a ball mill. The mixture was milled for fifteen hoursand then removed and the water was dried out. The resulting material wasscreened through a coarse screen. It was then calcined at a temperatureof 1100 degrees C. The calcined material was then ball milled again forfive hours. Thereafter a temporary binder, such as polyvinyl alcoholdissolved in water, was added. The mixture was then sieved to retain thematerial which passed through a mesh per inch screen but not through a325 mesh per inch screen.

The material obtained after the sieving operation was then pressed in acompacting press to a desired shape, such as 50 mils outer diameter, 30mils inner diameter, and 15 mils thick. The material was then fired.This was done by placing a thin layer of the shaped cores on a platinumsheet. The temperature provided was between 1050 degrees C., and 1350degrees C. This temperature was maintained for from one-half to livehours, depending upon the coercive force desired. The longer the heatingand the higher the temperature the. lower the coercive force.Thereafter, the cores were removed and air cooled.

Consider the formulations for 30 mil .cores respectively designated asA, B and C, which are shown below:

Table l Table 11 Core I V1 dVz t, t. i A 2 450 23 s .22 .44 .56 5.8 59035 e .22 .46 .71 s. 6 400 42 12 .22 .46 1.00 10. 0

I=full current drive in milliamperes. Vr=undisturbed one responsevoltage in millivolts. dVz=disturbed zero response voltage inmillivolts. t =core peaking time in microseconds.

ts=eore switching time in microseconds. A =stabilized delta noise inmillivolts.

=flux in volt-microseconds.

The terminology used as the heading for each column in Table II isintended to be the standard terminology which has been proposed andaccepted by the ASTM Committee C-25 for establishing standards fortesting and defining terms which apply to the use and testing ofnon-metallic magnetic cores.

The data found in in Table II is given for a temperature of 25 degreesC. at a full driving current as indicated with rise times at 0.1microsecond. Partial select current was set at 0.6 X'f-ull drive.Noteworthy for the new material are its low dV A (delta noise per pairof cores) and flux (at a nominal drive) which is approximately tenpercent greater than the test drive.

Some illustrations of formulations in accordance with this invention,used .to make 50 mil cores are shown in the following Table 1H:

T able Ill Core D as indicated, was driven with a 500 milliampere fu l-ldrive. A 300 milliampere partial drive was employed. A rise time of 0.15microsecond was achieved. Core E, the indium core, was driven with theindicated full drive of 450 milliamps, and a partial drive of 270milliamps was given. The electrical properties of these two 50 mil coreformulations are shown in Table IV.

Table IV Core I V1 (1V; tp l a where x varies from .05 to .15 mol andwhere H is a material selected from the group consisting of Mg SnO andMgIn O 2. An improved composition for a ferrite magnetic memory corewhich is suitable for use in magnetic memories, said compositionconsisting of where x varies from .05 to .15 mol, y varies from Zero upto .07 mol, and H is one of the materials selected from the groupconsisting of Mg SnO and MgIn O 3. An improved composition for a ferritemagnetic memory core which is suitable for use in magnetic memorics,said composition consisting of xMg SnO (.85-x) MgFe O .15Mn O where xvaries from .05 to .15 mol.

4. An improved composition for a ferrite magnetic core which is suitablefor use in magnetic memories, said composition consisting of xMgIn O(.85x)MgFe O.;- .15Ml'1304 where x varies from .05 to .15 mol.

5. An improved composition for a ferrite magnetic core which is suitablefor use in magnetic core memories, said composition consisting in mols:

.70MgFe O .15Mn O .15Mg SnO 6. An improved composition for a ferritemagnetic core which is suitable for use in magnetic core memories, saidcomposition consisting in mols:

.70MgFe O .15Mn O .05ZnFe O l0Mg SnO 7. An improved composition for aferrite magnetic core which is suitable for use in magnetic corememories, said composition consisting in mols:

References Cited by the Examiner UNITED STATES PATENTS 4/1962 SaccoOTHER REFERENCES TOBIAS E. LEVOW, Primary Examiner.

MAURICE A. BRINDISI, Examiner.

1. AN IMPROVED COMPOSITION FOR FERRITE CORES FOR USE IN MAGNETIC MEMORYSTORAGE DEVICES, SAID COMPOSITION CONSISTING OF